Getting Started

All FOCS files are plain text files, though some contain extended characters, especially the language files.
When editing them a somewhat more advanced editor is necessary. For example, on Windows, the Notepad and Wordpad editors can cause trouble, but the third party NotePad++ editor should be fine. An added benefit of using NotePad++ is that one of our contributors has made an add-on to help with the FOCS syntax.

When a reference is made to the default/ directory, this refers to the Resources Files directory.
This directory varies on different systems and can be manually changed.
If you are unsure where this directory is:
Start FreeOrion → Options → Directories (next to last) → Resource Files.

Creating a new entry

Create a new file in the default/scripting/buildings/ directory, name it TUTORIAL_ONE.focs.txt

This creates a very basic building type that could be built anywhere but has no effect.

No player would be able to build this building however, as their empire does not know how.
For now, we will just give the knowledge to everyone at the start of the game:

Edit default/scripting/starting_unlocks/items.inf

Add the following line at the top

Item type = Building name = "BLD_TUTORIAL_ONE"

If you start the game now and select the production screen at your home planet, you will see:

ERROR: BLD_TUTORIAL_ONE

Hover over that and the tooltip shows:

ERROR: BLD_TUTORIAL_ONE_DESC

This is because we have not told the game what our building should be labelled as, just to use the key reference of BLD_TUTORIAL_ONE.
Just like the filename, key references could be named anything, regardless of what label we want to use.
The name entry in the definition needs to be unique, but description can be shared among other entries if needed.

The key references are looked up based on the language selected, in one of the stringtables.
If an entry is not found, the game will use the default lookup as a fallback.
The default table is english, so we will just edit that one for now.

Edit default/stringtables/en.txt

Search for BLD_COL_SUPER_TEST_DESC

On the next blank line enter the following

BLD_TUTORIAL_ONE
A new building type from the scripting tutorial.
BLD_TUTORIAL_ONE_DESC
This is a brand new building type we just created.

Make sure there are blank lines before and after these new entries.

If you are planning to share these changes with others, always make sure any entries you create in other stringtables are included in the default stringtable.

You now have a very basic building in the game, though it has no purpose.

Keep in mind that this is just for demonstration. Building definitions should not be common structures built on every planet or more than once on any planet, in general. Structures like power plants or medical facilities would be covered by the infrastructure mechanic.

Entry Files

All of the FOCS entry files have a double extension of .focs.txt.
These files will be loaded regardless of the rest of their filename, or how deeply nested in the sub-directory they are.

Some special entry files have the extension .inf.
While most of these files can be freely edited, they can not be renamed or moved.

Any other file extension is not loaded into the game, unless an entry file specifically includes it.
The extension .macros is commonly used to denote scripting macros used in various entry files.
Entries that are not wanted in the game currently, but kept around for reference, are typically given the extension .disabled.

Entry files should never #include a file with another entry definition in it.

Directory Structure

Entry files are categorized by their type and should remain within the directory for that type (nested sub-directories are ok).
For example Tech entries can be anywhere in default/scripting/techs/, but should never be in default/scripting/fields/.
This only applies to the actual definition files, those with the extension .focs.txt.

The following directories within default/scripting are for .focs.txt entries:

alignments/

Alignment entries

buildings/

Building entries

empire_statistics/

Empire statistics entries

encyclopedia/

Encyclopedia topic entries

fields/

Field entries

monster_designs/

Ship loadout designs for space monsters

ship_designs/

Ship loadout design entries

ship_hulls/

Ship hull designs

ship_parts/

Ship part entries

specials/

Entries for specials

species/

Species entries

techs/

Tech entries

Required files

These are relative to default/scripting/:

keymaps.inf

To Be Determined - Default keymap values

monster_fleets.inf

Entries for monster fleets

starting_unlocks/buildings.inf

Buildings pre-built on every starting homeworld.

starting_unlocks/fleets.inf

Entries for fleets every player starts the game with.

starting_unlocks/items.inf

Blueprints available at the start of the game to every player. For techs this means they are completed, for other types it allows the player to produce them.

techs/Categories.inf

Categories that a tech must belong to. For now these should not be changed or added to, aside from possibly the graphic or colour definitions.

Additionally, definitions listed in any starting_unlocks/ file or monster_fleets.inf need to be avaiable.
The default AI may assume any or all of the default entries to exist.

Macros

A basic macro definition looks like this:

FIRST_ONE
'''2'''

The three apostrophies denote preformatted text, meaning any newlines are part of the macro.
Macro names should be in all uppercase, with underscores in place of spaces. They should also be uniquely named.

Macros are mainly used to keep consistant values for multiple definitions.
When this is the case, it is best to have the macro definition in a file by itself, so other files can include it.
Sometimes macros are used for a complicated formula to help the readablity of the file. There is no need for these macro definitions to be separated if they are never used in another definition.

Including other files

Files are usually included for macro definitions.
You can include another file with the #include directive:

#include "some_file.macros"

Includes the file named some_file.macros

The directive should by on a line by itself, without any spaces before #include.
There is no restriction on what files are included, however you should not include files containing another FOCS entry in it. This would lead to a conflict when the entry is loaded twice.

Any path separators in the filename should use the forward slash ( / ) and not the backslash ( \ ).
If the filename begins with a path separator, it will refer to the Resource Directory, and not the file-system root.
The double-dot token ( .. ) will refer to the preceding directory, this should be used sparingly as it is hard to read (and easy to mistype) a filename like ../../../../ship.macros.

There is also special handling for a leading asterisk ( * ) in the name of the file, for wild-card matching.
This is useful for including a batch of files from a common place.
Placement of an asterisk outside of the first character, and the use of the single character wild-card ( ? ) are not supported.

Any specific file is only included once per file. When editing a file, it should include any files it needs for macros and not depend on inclusion within other files.

Examples

For the following examples, say we are editing a new file default/scripting/specials/TIMID_SHIP_AI.focs.txt:

#include "/scripting/abc.123"

would point to default/scripting/abc.123.

#include "../common/abc.123"

The above line would refer to default/scripting/common/abc.123.

#include "common/*_AI.macros"

Will include all of the files in default/scripting/fields/common (and any sub-directories) that end with _AI.macros.

This might include the imaginary files default/scripting/fields/common/SHIP_AI.macros and default/scripting/fields/common/BASE_AI.macros, but not default/scripting/fields/common/SHIP.macros.

Troubleshooting

Always check your log files when working with FOCS files.

Typically if there is a problem with the game parsing an entry, it will show the error in the console window.
The error will also be in the log file freeorion.log, which is located in your user directory (Options > Directories > Save files (parent directory))

If there are no errors shown, you can make sure the file is loading by setting the log level to TRACE by either:

editing the config.xml file in your user directory and change <log-level>VALUE</log-level> to <log-level>TRACE</log-level>

launching the game with the --log-level TRACE argument.

The client log file (freeorion.log) will now show each file as it was loaded in, as well as any files that were skipped.

If the issue is just with the name or description of the entry (it shows as ERROR: ENTRY_NAME in-game), make sure the entry is correct in the english stringtable (default/stringtables/en.txt).

If you need further help, post a question in the forums, attaching your log file will probably speed up any response.

Adding New Content

Linking a Tech and a Building

Create a new file in default/scripting/techs/
We will save this file as TUTORIAL_ONE.focs.txt

Requires DEF_ROOT_DEFENSE to be researched first (which every player starts with)

Allows a player to build BLD_TUTORIAL_TWO (we have not created this yet)

Increases the speed of new troop recruitment on the planet by an additional 1 per turn, as long as combat is not occurring.

The short description is for the hover text on the tech screen, as well as the brief description at the top of the 'pedia entry.
Techs all belong to one tech category, these are defined in Categories.inf. It is recommended to use one of the existing categories.

Notice researchcost uses the macro TECH_COST_MULTIPLIER, which is defined in default/scripting/common/shared.macros So we also #include that file at the bottom.
The ../ means to go back one directory, in this case to default/scripting (because our file is in default/scripting/techs).
This definition also uses the CANDIDATE_BATTLE_CHECK macro, so the techs.macros file is needed for that.

This building can only be built on a planet with at least 1 population, and is limited to one per planet.
It also has a couple of effects, the first allows an additional 3 troops on the planet
The second allows an extra defense of 5, but only if there is also a BLD_TUTORIAL_ONE on the planet.

location tells the game where this building can be constructed.

check all of the planets

for each planet that is without a BLD_TUTORIAL_TWO

that is also owned by the player

that also has a population of at least 1

EnqueueLocation restricts when a building can be placed into the production queue.

The effectsgroups contains any effects and the conditions of when and where to apply them.
There are a number of different ways to construct these, so we will show two different methods.

Looking at the first EffectsGroup, the scope defines the Target of our effect:

check all of the planets

for each one that has an ID that is the same as the sources(this building) planet ID. ID is a unique identifier, so this will only match one result.

For the second EffectsGroup:

check all of the planets

for each one with the same owner as the sources(this building) owner.

that has a completed BLD_TUTORIAL_TWO

and also has a completed BLD_TUTORIAL_ONE

effects define what effects apply. The Target variable can be used here if needed, but it can not be used in scope or activation because they define what the target is.

For a more in-depth look at all of the possible attributes and descriptions of each, see Effects

Remember to add in the stringtable entries for the new tech and building:

Start a new single player game with 1 AI player and Planet Density set to High.

If you play with this building for awhile, once you have it on multiple planets you may notice an issue.
Wherever we have both buildings constructed, it increases the defense effect for the buildings on all of our planets.
That might be a good feature for some other building, but it is not really what we intend here.

Once you have these two buildings on a couple of planets, save the game and exit before we make further changes.
The game will load the FOCS files into memory, so changes made will not neccesarily take effect until restarted.

One way to try and solve this is by adding a stacking group to the effect.
Change the effectsgroups of BLD_TUTORIAL_TWO so it looks like this:

stackinggroup only allows the first instance of tha effect to apply, ignoring any others. You might create a new building or tech with the same stackinggroup name, and only the first one would apply.

If we gave both effects the same stackinggroup, only the troop effect would apply.

After saving the changes, load the previous game and click on next turn.
The game state is loaded as we left it, once we start a new turn everything is reprocessed.

Now you will see only one defense effect at each planet. While this functionally works how we want, if you hover over a couple of planets they show the effect all coming from the same planet (if they have a supply connection).
What we really want is for the effect to only apply to the planet it is built at.

Our first effect works fine for this. We could combine the effects onto one effect group, saving us from doing the same scope check twice:

If returns either a 1 when true, otherwise is returns a 0.
This would show a bonus of 0 for defense when we do not have a BLD_TUTORIAL_ONE constructed. If everything showed a notice for not having an effect, the user interface would be very cluttered.

Instead of showing useless information, we will just modify a copy of the first EffectGroup.default/scripting/buildings/TUTORIAL_TWO.focs.txt

Everything should be working as we had before. When you are testing your changes you can destroy a building by right clicking on the icon for it, on the planet side screen.

We have left the icon blank in all of our examples, letting the game decide the default icon to use. Where mentioned, icon or graphic are required fields and wll be relative to the art directory (default/data/art/). If sharing these wih others, you might pick the icon or graphic asset that best suits the entry.

Ships

Any single ship is constructed from a ship design, which is a combination of a ship hull and zero or more ship parts.

The previous section introduced a number of errors to show ways to troubleshoot them. The remaining sections will concentrate more on anything specific to them. Stringtable entries will be ommitted to keep the sections briefer, feel free to add them as appropriate.

Ship Parts

Ship Part Class

ShortRange

FighterBay

FighterHanger

Shield

Armour

Troops

Detection

Stealth

Fuel

Colony

Speed

General

Bombard

Research

Industry

Trade

ProductionLocation

Keep in mind, these parts will not be available to an empire until they unlock them either by researching a tech, getting them at the start of the game, or through some other means.

You can see what parts are in the game, but not yet unlocked by your empire, by going to the ship design screen and selecting Unavailable in the parts filter.

class defines what kind of part this is: a short range weapon, a fighter weapon, a piece of armor, etc.
For direct fire weapons (ShortRange), there is a primary stat of damage and a secondary stat of shots.damage refers to the amount of damage for each shot.shots is how many times this part shoots during each combat round, this may be omitted and defaults to 1.
The FighterHangar class has a primary stat capacity and a secondary damage.

mountableSlotTypes are slots types that this part will fit into. For balancing purposes: weapons, armor, and sensors should usually be limited to the outside of the ship, with other types restricted to the interior. There are exceptions to this, such as troop pods, but the majority stick to this guideline.
For ship designs, the buildcost of each part and the hull will be accumulative. However the buildtime is not, the longest time of all of the parts will be used.location limits where ships designs with this part may be constructed. Some part might be restricted to planets with access to a specific special, or a certain system type.

Many parts simply have a capacity. What that capacity relates to depends on the class of the part, in this case it is the amount of extra maximum structure a ship design has.
This allows parts to have effects applied on them from other events with the SetCapacity and SetMaxCapacity effects.

speed, fuel, stealth, and structure are all starting values.slots has an entry for each slot and the coordinates to display that slot on the graphic during ship design.
Since the position of the slots need to line up with the graphic used, we will just use the basic large hull here.

parts need to stay in the same order as they are defined in the hull.
The first three are all external slots, and the fourth is an internal slot.
Since our new armor part can fit in an internal or external slot, this is ok.

Ship designs within the Resource Files are available to all empires.
Designs saved from inside the game are saved in the User Directory, and only available to the human client.

There are also designs for space monster ships, which follow the same layout as other designs.
They are in a separate directory as the fleets are handled differently.

Fleets

Player fleets are typically only defined within default/scripting/starting_unlocks/fleets.inf
These encompass what ships each player starts the game with, and how they are grouped.
A sample fleet:

spawnrate defines how often the space monster might appear. This is factored with other values, including the frequency set by the player.spawnlimit defines the maximum number allowed in the game. The monster would not spawn anymore during the current turn, if there are more than this limit.

String Tables

String tables are the language files used when displaying information to a user.

These files are located in default/stringtables/ and are separated by language, the default being the English file en.txt.
Each client has a language setting, which determines which file it uses for references. Any references not found are then checked for in the default file.

When an entry is given a name="BLD_ONE", anytime the game passes that entry around it uses BLD_ONE.
When the game displays information about it to the user, the users client translates it according to their string table.

Given the entry in en.txt:

BLD_ONE
Building One

Any time the building name is displayed to the user, they will see Building One.

Macros

Stringtable Macros

Type

Value

planet

ID

system

ID

fleet

ID

ship

ID

building

ID

empire

ID

tech

string

buildingtype

string

special

string

shiphull

string

shippart

string

species

string

encyclopedia

string

Macros in string tables are not handled exactly the same as macros in other FOCS files.

Like a normal macro, you can use a string table entry to swap the value for the text.

[[BLD_ONE]]

swaps [[BLD_ONE]] with Building One

Alternatively, macros may have a preceding type followed by a value which is a string table entry or an ID.
These string table macros are used to create a link to a 'pedia entry.

[[buildingtype BLD_ONE]]

swaps [[buildingtype BLD_ONE]] with a link to the BLD_ONE pedia entry displayed as Building One.

Each of the string table entries are loaded into their type, such as tech. To create the correct link you need to know which type they belong to.
If the wrong category is used, a blank 'pedia entry will be shown when the link is clicked on.

label is a reference to a string table entry. It is used for filtering sitrep messages, sitreps can share labels.

Each tag in the parameters is passed as a variable to the message.
The message is a string table entry, the variables are referenced with surrounding percentage signs (%).

CUSTOM_SITREP_INTRODUCTION
'''A custom introduction that references the tech %tech% and the system %system%.'''

These variables are shown as links to the item or pedia entry.

Variables are predefined, a custom variable will not work.
The variable text will be translated from the supplied userstring.(string table match)rawtext will not be translated and will be displayed as defined.

empire defines which empires will receive this message. It is optional, and if omitted the message will be shown to all empires.

General purpose sitreps that should be shown at the start of the game should be placed into the default/customizations/custom_sitreps.txt file.
There are a number of examples for different sitrep options in this file as well.

Encyclopedia Articles

The encyclopedia articles are 'pedia entries which would not belong to a specific entry.
These include guides, game concepts and overviews for groups of entries.

The definitions are placed in default/scripting/encyclopedia
Except for icon, all of the fields are string table entries.

Fields

Fields are defined in default/scripting/fields, and represent effects that span a limited area in space.
While many of these entries are created during universe generation they can also be created as an effect of another entry using CreateField.

effects = CreateField type = "FLD_ACCRETION_DISC" size = 10

Specials

Specials are items which can be attached to most of the other universe objects, typically they are found on planets.
They are defined in default/scripting/specials

Similar to monster fleets, they have a spawnrate field and an optional spawnlimit field that will limit how often they will occur.

Species

A good amount of info is packed into the effects for species entries. Historically this was for processing efficiency and to ensure the ordering of specific effects.

Following Effects(Species), note the fields for Playable, Native, CanProduceShips, and CanColonize.
The default for these attributes is false, meaning if they are not listed in the file, the species can not perform that action.Playable determines if the player can select them as their starting species, and if the species is available to the AI.Native species are distributed among planets at the start of the game, according with the game-setup options. They are not controlled by an AI client and so do not typically construct buildings or ships.

foci contains each planet focus type that the species is able to select from. This does not give the species the knowledge of that focus type, only the ability to use it if they later acquire it.

preferredfocus is the focus type that will give certain bonuses, like increased happiness, to that planet when used.

In addition to having CanProduceShips and CanColonize in the species entry, the species will need a colony building entry and knowledge of how to construct them.
Entries for other species colony buildings are in default/scripting/buildings/colonies. For official additions, the col_bld_gen.py script should be edited instead and generated from that script.
This file is a Python script and will need the expertise of someone that can program in that language.
Most colony buildings are granted knowledge of to all players at the start of the game from default/scripting/starting_unlock/items.inf.
Some of the species, such as any of the extinct species, are not included here, but granted after a tech is researched or some other criteria is met.

Example

The following will be a step by step instruction on "quickly" creating a new playable species.

Make sure you are using a decent text editor before starting (mainly not Notepad or Wordpad, see above).

Change to the (path to freeorion)/default/scripting directory.
Copy species/SP_HUMAN.focs.txt to species/SP_NEW_MAN.focs.txt

Open this new file and change the following fields:

name = "SP_NEW_MAN"

description = "SP_NEW_MAN_DESC"

Next, make sure to create a colony building for this species:
Copy buildings/colonies/SP_HUMAN.focs.txt to buildings/colonites/SP_NEW_MAN.focs.txt

Open the new file and replace the any human entries with entires for newman. Replace:

"BLD_COL_HUMAN" with "BLD_COL_NEW_MAN"

"BLD_COL_HUMAN_DESC" with "BLD_COL_NEW_MAN_DESC"

"SP_HUMAN" with "SP_NEW_MAN"

These new entries need to be creating in the stringtables:

Open the default stringtable: (path to freeorion)/default/stringtables/en.txt.

Find the entry for SP_HUMAN (CTRL+F in most editors).

On the line just before SP_HUMAN, enter the following:

SP_NEW_MAN
Newman
SP_NEWMAN_DESC
A new species with no differences from Humans

Note that we did not change the gameplay_description field in the species entry, normally each species should have a new stringtable entry here.
For now, both the Human and Newman species show the same description on the Single Player galaxy setup screen.

Order of Events During the Turn

It can be helpful, sometimes important, to understand the order of event handling by the server. The following summarizes the most pertinent parts of the sequence of server processing that starts once all players have ended their turn.

orders are executed (invasions & colonizations done, fleet movement orders set, research & production queue changes set (but the progress on those queues is not yet handled)

fleets_move

empire visibility is updated & a pre-combat turn update is sent to players

combat situations are determined and combat takes place

empire visibility updated again

all effects applied, including building new "current" meter values from "initial" meter values

empire visibility updated again

blockades, supply & resources updated

things made, techs researched

scripted events processed

meter effects applied again (rebuilds new "current" meter values from "initial" meter values, not adding another dollop to current meter values)